1. Field of the Invention
The present invention relates to image sensing apparatus for sensing an image of an object and a method of controlling the image sensing apparatus.
2. Related Background Art
It is known that, when fluorescent substances of certain kinds are exposed to radiations (X-rays, α-rays, β-rays, γ-rays, electron beams, ultraviolet rays, etc.), part of the radiation energy is accumulated in the fluorescent substances and that, when such fluorescent substances are then exposed to scintillation light such as visible light or the like, the fluorescent substances exhibit stimulated emission according to the accumulated energy. The fluorescent substances possessing this property are called storage type phosphors (photostimulable phosphors).
There are proposals of radiographic information recording/reproducing systems making use of such storage type phosphors and configured to record radiographic image information of an object such as a human body or the like once in a sheet of an storage type phosphor, scan this storage type fluorescent sheet with scintillation light such as laser light or the like to cause stimulated emission, then photoelectrically read the resultant stimulated emission to obtain image signals, and present a radiographic image of the object as a visible image on a recording material such as a photosensitive film or the like, or on a display device such as a CRT or the like, based on the image signals (e.g., in Japanese Laid-Open Patent Applications No. 55-12429, No. 56-11395, etc.).
There are also systems developed recently to sense an X-ray image similarly by use of semiconductor sensors. These systems have the practical advantage of capability of recording the image covering an extremely wide radiation exposure range, as compared with the conventional radiographic systems using silver halide films. Namely, such systems are configured to read X-rays in a very wide dynamic range by photoelectric conversion means, convert them into electric signals, and output the radiographic image as a visible image on the recording material such as the photosensitive film or the like or on the display device such as the CRT or the like, using such the electric signals, whereby the systems can provide the radiographic image without being affected by variation in exposure dose of radiations.
Operation cycles of the conventional X-ray sensing apparatus are normally one-day periodicity. For example, upon an operation test of an X-ray generator, the power is also turned on for such devices as an X-ray film changer, an X-ray solid-state imaging device, and so on, thereafter the power is kept on during hours when objects, e.g., patients can appear, and the power is turned off after an end of the last image sensing in that day.
It is very rare for the image sensing apparatus to sense X-rays incessantly during the hours. It is thus common practice to control the sensing apparatus to low power consumption during non-sensing periods or to bring the system into a standby mode in which the imaging device is liberated from an imaging state so as to reduce the load on the imaging device. For implementing it, for example, an operator provides a command to transfer the system into the standby mode, or the sensing apparatus automatically moves into the standby mode if there is no access to the sensing apparatus for a predetermined period.
When an object, e.g., a patient appears, the apparatus normally shifts from the standby mode into the normal imaging mode in response to a command inputted by the operator.
Japanese Laid-Open Patent Application No. 10-104766 discloses the prior art in which a patient detecting sensor is provided in proximity to the X-ray image sensor, the X-ray image sensor is kept in the sensing state (on) only during a period when a patient is present in front of the sensor, and the sensor is turned into the standby state (off) during periods when no patient is present.
Since the conventional apparatus makes transitions between the sensing mode and the standby mode according to instructions from the operator, as described above, the apparatus can be maintained in a state of preparation for sensing in spite of absence of a human body (object) because of an operation error of the operator, a long setting of the predetermined period, or the like. This can sometimes result in decreasing the lifetime of the X-ray sensing device. Sensors comprised of semiconductors can suffer problems of occurrence of a wait time during the preparation for sensing or before sensing, and decrease in the total product life. Energization of the sensing part for a long period will result in generating excess heat in the sensor housing and the heat can bring about a demerit of increasing offsets of the sensor or a read-out circuit.